I read it as purely figurative. The cannibals are the Axis, missionaries are allies
At first glance, that too was my impression but then I recalled reports of non-survival cannibalism by the Japanese. This then made me wonder if the British officer may have heard such reports coming from China and his choice of words was not entirely figurative?
In 1942 when the supply situation for the Japanese was still good, Australians began encountering evidence of cannibalism by Japanese troops in New Guinea. http://www.pacificwar.org.au/JapWarCrimes/TenWarCrimes/Murder_Cannibalism_Kokoda.html
That the Japanese in the Philippines were cruel is a fact but as Japanese troops began to be withdrawn from New Guinea to the Philippines later in the war, Filipino guerrillas reported that the Japanese from New Guinea were exceptionally savage.